In primate brains, only mammalian GnRH (mGnRH) has been detected. Here we report the presence of a second form of GnRH, chicken GnRH-II (cGnRH-II), in the monkey brain. First, four brains from rhesus monkey fetuses at E72-E77 were powdered, combined and extracted with acidic acetone. HPLC and RIA analysis showed the presence of mGnRH (antisera GF-4 and B-6) in fractions 20/21. This elution position matched that of the synthetic peptide. Second, 3 adult brains from stumptail monkeys at ages 14-19 years were combined. Both mGnRH cGnRH-II were detected; cGnRH-II (antiserum 7CR10) was in fraction 26. Third, the diencephalon-medial telencephalon part had the highest content of mGnRH; the midbrain-brain stem and diencephalon-medial telencephalon had equal amounts of cGnRH-II; the cortical regions including the temporal lobe had the lowest content of both peptides. An additional peak of GnRH of unknown structure was detected by antiserum B-6. Fourth, immunocytochemistry with antiserum 675 (J. King and R. Millar) showed the presence of cGnRH-II positive cells in the periaqueductal regions of the midbrain in a rhesus monkey fetus at E85 and in regions surrounding the mesencephalic and rhombencephalic vesicles in a rhesus fetus at E38. Fifth, injection of cGnRH-II into adult rhesus monkeys N-5) resulted in a significant increase the circulating plasma LH concentration at 10 min after injection during the luteal phase, but no change after injection in the follicular phase. We conclude the cGnRH-II as well as mGnRH are present in the monkey brain and that cGnRH-II is capable of releasing LH during the luteal phase. (Findings from this study were presented in 1996 at the International Congress of Endocrinology, held in San Francisco, CA).